1. Field of the Invention
The present invention relates to a communication system, especially to a communication receiving end of a wireless communication system and an auto gain control method thereof.
2. Description of Related Art
FIG. 1 illustrates a block diagram of a conventional signal receiving end. Because the signal power received at the receiving end is unknown, the receiving end has to rapidly complete the adjustment of the gain of the signal amplifier 110 so as not to interfere with the reception of the signal. The success rate of the cell search is subject to the speed at which the gain control unit 140 adjusts the gain. Only when the cell search is complete, the post-stage circuit 150 then processes the inputted signal that is already converted to a digital format. In a conventional gain adjusting process, the power estimation unit 130 calculates the power of the inputted signal, and then the gain control unit 140 compares the obtained signal power with a reference power to obtain a gain difference, which is subsequently fed back to the signal amplifier 110. When the analog-to-digital converter (ADC) 120 becomes saturated (i.e., the gain of the signal amplifier 110 is too large), and no other information is available to obtain an appropriate gain, a converging process by a progressively decreasing approach is employed to obtain the appropriate gain—this mechanism is known as a close-loop control mechanism. However, in a time-division duplexing long-term evolution (TDD-LTE) system, the configurations of uplink (UL) and downlink (DL) are unknown before synchronization. The powers of the UL signal and the DL signal differ greatly, in a way that the signal passing the signal amplifier 110 may saturate the ADC 120 if the gain of the signal amplifier 110 is too large or that the signal passing the signal amplifier 110 becomes so small that the converted signal generated by the ADC 120 fails to precisely reflect the content of the signal if the gain of the signal amplifier 110 is too small. Further, in a frequency-division duplexing long term evolution (FDD-LTE) system, different numbers of resource blocks (RBs) may be used in different DL subframes, causing the power of a received signal to vary from subframe to subframe. For the above reasons, the conventional gain controller that employs the close-loop control mechanism has to adjust the gain back and forth for several times before the gain converges; this slow and repetitive procedure decreases the success rate of the cell search dramatically.